The long-term goal of this project is to understand why influenza B changes more slowly (and probably in a different mode) compared to influenza A, yet causes as many epidemics in the human population. An understanding of the molecular basis of the difference should lead to more effective vaccine strategies. The neuraminidase has been selected for study since its structure determination is under way, since it has an enzyme activity which can be quantitatively measured, and since its activity can be replaced by exogenously-added enzyme. The experiments will investigate the relative importance of selection of antigenic variants, accuracy of replication, selection to retain NA function, and selection for optimal base composition or codon use, in variation of influenza B viruses compared to influenza A. The specific aims are: 1. To characterize antigenic sites on the neuraminidase (NA) of a recent type B epidemic virus, B/Memphis/3/89, and combining sites on anti-NA antibodies. Epitopes will be characterized using monoclonal antibodies and identification of sequence changes in antibody-selected escape mutants, and further delineated by site-directed mutagenesis of both antigen and antibody. 2. To determine if the lower rate of change in influenza B is established by the influenza B polymerase system. This experiment takes advantage of newly-developed technologies to package foreign genes into negative-stranded RNA viruses. The B/Mem/89 NA gene will be placed in an influenza A background to determine if the slow rate of change is a property of the B replication system, or if it results from constraints on the protein or its gene. 3. To identify and compare other evolutionary pressures on the influenza A and influenza B NA protein and gene. To investigate evolution of the protein, sequence changes will be determined in the influenza A and B NA when the viruses are grown under conditions where there is no requirement for NA function. Evolutionary pressures on the gene will be examined by replacing the influenza NA gene with a bacterial NA gene which has a very different base composition.